1. Field of the Invention
The present invention relates to a semiconductor memory device, more particularly, to a semiconductor memory device with improved stability of operation in each bipolar memory cell having Schottky barrier diodes as loads.
2. Description of the Related Art
In a semiconductor memory device, it is necessary that the contents of the memory in a memory holding state not be easily changed due to alpha rays or other external noise. To stabilize the memory holding state, the voltage difference between the base and the collector of a transistor in a memory cell may be increased when the transistor is in an OFF state. The voltage difference between the base and the collector of the OFF-state transistor is determined by the product of a holding current flowing through the memory cell during the memory holding state and a load resistance of the transistor.
There are, however, problems with this. That is, if the load resistance is too large, the operating speed of the memory cell is lowered. If the holding current is increased, the power consumption of the overall semiconductor device is increased. Also, when a Schottky barrier diode is connected in parallel with the load resistance for preventing oversaturation of the transistor, the above-mentioned product of the holding current and the load resistance must be lower than the clamp voltage of the diode. The clamp voltage of the diode cannot be made higher than a predetermined value, for example, 0.4 V, to ensure the stability of the memory holding state.
As prior art in this field, Japanese Unexamined Patent Publication No. 53-43485, published on Apr. 19, 1978, Applicant: Hitachi Ltd., discloses a semiconductor memory cell in which a capacitor is connected in parallel with the Schottky barrier diode. The capacitor is provided for achieving high speed operation. In this reference, however, a highly doped diffusion region is used for obtaining both the Schottky barrier diode and the capacitor. Therefore, the forward voltage V.sub.F of the Schottky barrier diode is too small, and the stability of the memory cell is not sufficiently high.